The unidirectional pattern of gas flow in the avian lung is directed by aerodynamic valves. Recent studies suggest the fluid dynamic mechanism by which inspiratory flow is directed, but the mechanism of expiratory flow direction is yey unknown. Expiratory flow in the bird appears to be limited by mechanisms similar to those causing expiratory flow limitation in mammals, but is sited in the syrinx, the organ of vocalization. Sound production (honk) is coincident with flow limitation. Massive flight muscles insert on the sternum, yet there is evidence suggesting that respiratory function of the chest wall is mechanically independent of winS beat. We propose: 1) to investigate the smooth muscle control of airway features governing aerodynamic valves; 2) to characterize the aerodynamic valving of expiration using experiments in animals and physical models together with mathematical analysis of pertinent fluid dynamics; 3) to examine the performance of aerodynamic valves at the high respiratory frequencies associated with thermal panting; 4) to characterize sound production and flow limitation in the syrinx, testing their dependence on gas density; 5) to describe the respiratory consequences of wing stroke, and to model the mechanical interaction of the chest wall and flight muscles. Normal activities such as exercise, flight, performance at altitude, thermal stress and panting and vocalization each present a unique challenge to the avian respiratory system. These proposed studies address problems in airway fluid dynamics and ventilatory mechanics pertinent to these challenges. Moreover, the forces governing avian aerodynamic valves also become important in mammals at high ventilatory flow; the normal relationship between sound production and flow limitation in the syrinx resembles hypothesized mechanisms of wheeze production in humans; the interaction of wingbeat and respiration is related to the mechanical coupling between arm or leg tasks and respiration. These are enlightening points of comparison between the mammalian and avian respiratory system.